Blowdown is water discharged from boiler systems containing a relatively high concentration of suspended and dissolved solids. The discharged blowdown is replaced by fresh (usually demineralized) low-solids feedwater. Excessive solids buildup in boiler water can cause carryover into the steam drum in the absence of suitable an-tifoam agents, or scaling when salt solubility is exceeded. Boiler blowdown water is generally alkaline and often contains suspended matter from sludges of insoluble sulfates and carbonates. High temperature, dissolved solids, and alkalinity present disposal problems for untreated blow-down water, unless a larger wastewater stream is available for dilution before discharge into receiving water (IUPAC 1963). For petrochemical plants, the blowdown stream is a minor contributor to the overall plant effluent disposal problem (Beychok 1967).
The rate of blowdown at equilibrium must ensure that solids introduced into the boiler by the feedwater are totally removed. Since chlorides are soluble and none are intentionally added to the boiler feedwater, they provide a means for measuring total salts in boiler water. If chloride and total soluble salt concentrations in the feedwater are known, the ratio of feedwater chloride to boiler water chloride indicates the ratio of feedwater total salts to boiler water total salts. The equilibrium total dissolved solids allowable in the boiler water are a direct function of the operating pressure of the steam-generating system (De Lorenzi 1951). A simple equation for determining the required rate of blowdown is given by Equation 8.4(1)
X=% blowdown (based on steam produced) or
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